At facilities where the use of nitric acid is widespread, large amounts of alkaline, nitrate-based wastes are generated. Nitrate, as a waste species, is very mobile in the environment and cannot be held up very well in any type of material, including cement-based grout to bitumen. For this reason, prior art abatement methods involving conversion to a gaseous form have included thermal degradation and treatment with chemical reductants at low pH.
The use of the electrolytic cell has shown the most promise of the known methods, except that it creates a new problem in the co-production of large quantities of sodium hydroxide.
U.S. Pat. No. 4,642,192 to Heskett discloses a method of treating fluids, in which waste water containing inorganic species such as chlorine, in the form of hypochlorite or as nitrate, is passed through a bed of metal particles to reduce the chlorine to chloride and the nitrate to a form not mentioned. While the data tables of the patent indicate that nitrate is very slightly reduced, the reduction is inefficient. No disclosure is made of process parameters that would produce an efficient reduction process.
U.S. Pat. No. 4,695,447 to Shutlz discloses a method of destroying inorganic hazardous wastes. Oxides of hazardous metals and non-metals are added to molten aluminum to cause the metals to be either reduced to a metallic state or dissolved in the aluminum or volatilize from the melt, depending on the species. While it is mentioned that nitrate is reduced to nitrogen, it is likely to occur through thermal decomposition since molten aluminum, which is used in the process, has a melting point of 660.degree. C.
U.S. Pat. No. 3,542,657 to Mindler et al discloses a standard electrochemical cell reduction method in which sodium nitrate in alkaline solution is reduced. However, the electrolytic cells reduce nitrate while co-producing vast amounts of sodium hydroxide. Generally, cell operation and stoichiometry requires the co-production of 40 pounds of sodium hydroxide for every 85 pounds of sodium nitrate reduced, according to the following equation: EQU 3 NaNO.sub.3 +3 H.sub.2 O--NH.sub.3 +N.sub.2 +3NaOH+9/2 O.sub.2.
Production of sodium hydroxide in these quantities has discouraged use of electrochemical cells.
Another problem with electrolytic cells, in addition to high capital costs, is that the presence of an anode in the circuit can reoxidize reduced chemical species and drop cell efficiency as a result from nearly 100% down to as low as 30%. For example Cr is a very problematic species since, at concentration greater that 1 ppm, the cell efficiency begins to drop rapidly. At 36 ppm Cr, efficiency drops as low as 30% at a very high electrical cost. Additives, such as Bismuth, have been employed to overcome this problem, as disclosed in U.S. Pat. No. 4,632,737 to Mindler.